1. Field of the Invention
This invention relates to an optical element, an optical device, and a display device.
2. Background Art
Recently, rapid development has been made in high-functionality devices such as electronic devices, sensors, and actuators based on MEMS (microelectromechanical systems) technologies. Application of MEMS technologies to optical equipment such as display devices and scanners includes a technique for using a MEMS device to modulate incident light for each pixel. In this case, the MEMS device serves as a switching device for switching on/off a lightwave.
Mechanisms for modulating a lightwave are classified into two types: the mirror type, such as DMD (digital mirror device), and the grating type, such as GLV (grating light valve). The DMD is based on a reflecting mirror having a size of e.g. approximately 15 to 25 μm, which can be tilted back and forth by approximately 10 degrees so that the direction of the optical axis of the reflected light is variable. However, because the DMD needs a mechanism for tilting the reflecting mirror, the structure of the hinge supporting the mirror surface is complicated and increases the manufacturing cost. Many problems remain to be solved with regard to such aspects as manufacturing yield.
On the other hand, an example of the GLV is disclosed in D. Bloom, “The Grating Light Valve: Revolutionizing Display Technology”, Projection Displays III Symposium, SPIE Proceedings Volume 3013, February 1997. This GLV has a structure in which ribbon-shaped grating elements are aligned on a silicon substrate. The grating elements alternately include fixed ones and ones that can be bent downward by electrostatic attractive force. In the state without bias application, all the grating elements are coplanar and do not generate diffracted light. On the other hand, upon application of a bias, movable grating elements are bent downward to form a concavo-convex surface in conjunction with the fixed grating elements. This concavo-convex surface diffracts light to generate diffracted light.
A MEMS device based on such grating elements has an advantage of being able to apply large optical modulation with small mechanical displacement and being capable of rapid response. Furthermore, such a MEMS device has high mechanical reliability. Hence, it has been applied to such devices as display devices, printer scanners, and gain equalizers for optical communication.
However, in conventional DMD or GLV gratings and display devices based thereon, the light source needs to be spatially spaced from the elements for mechanically switching a lightwave and to be provided with a certain incidence angle with respect thereto. This results in a large system volume occupied and limits downsizing.